Organosilicon compounds having a hydrolyzable silyl group and an organic reactive group are generally known as silane coupling agents and often used as adhesives, paint additives and resin modifiers, for example, since they are capable of forming bonds between inorganic materials and organic materials. The problem associated with the use of a silane coupling agent in hydrolyzate form is that it contains a high proportion of a volatile organic solvent which is added as the stabilizer/diluent for an alcohol and silanol resulting from hydrolysis of hydrolyzable silyl groups as typified by alkoxysilyl groups.
In general, silane sol-gel coating materials having a certain level of storage stability may be prepared using partial hydrolyzates of alkoxysilanes and acid-based hydrolytic catalysts. However, these coating materials are limited to alcohol and organic solvent based systems. If the system uses a sufficient amount of water to complete hydrolysis of alkoxysilanes and has a high solid content, then the system undergoes a substantial loss of storage stability and becomes difficult to control hydrolytic condensation of alkoxysilanes in the course of coating material preparation, leading to a molecular weight buildup or gelation.
JP-A 2009-524709 discloses a water-dilutable sol-gel composition essentially comprising glycidyloxypropylalkoxysilane, aqueous silica sol, organic acid, and organometallic compound of titanium or zirconium, which serves as a low alcohol content, aqueous siloxane coating material. With respect to the applications of this composition, only corrosion control coats and primer coats are referred to while its effectiveness is demonstrated nowhere. This coating material leaves a concern that organic acids may remain as the volatile organic compound.
Typical organic reactive groups on silane coupling agents include vinyl, amino, epoxy, (meth)acrylic, mercapto, isocyanate, ketimine structure, and styryl groups. Silane coupling agents having such groups are well known and used in a variety of applications.
Among others, silane coupling agents having an amino group contributing to water solubility have not only a high water solubility, but also a high aqueous solution stability so that an alcohol-free aqueous silane solution may be prepared by removing the alcohol resulting from hydrolysis. They are expected as less environmental load material.
JP-A 2012-046576 discloses that a silanol type silane coupling agent having a dicarboxylic acid structure which is obtained from hydrolysis of an acid anhydride-containing silane coupling agent is useful as modifier for encapsulant epoxy resins. No reference is made to the alcohol generated from the coupling agent. This technology falls in the range that silane coupling agents are used in hydrolytic aqueous solution form.
On the other hand, mercapto-containing silane coupling agents are believed effective for adhesion to metal substrates. An aqueous solution of the mercapto-containing silane coupling agent can be prepared as long as the concentration of the agent is as low as 0.1 to 5% by weight, and a water-miscible organic solvent (or volatile organic compound) such as methanol is co-present. However, preparation of an aqueous solution of the agent to a high concentration and reduction of the alcohol generated are inhibited by the high hydrophobicity of mercapto groups.
As discussed above, silane coupling agents having hydrophilic groups such as amino and carboxylic acid groups can form aqueous solutions having high stability whereas silane coupling agents having other organic functional groups, especially mercapto are difficult to form aqueous solutions with high concentration. Those materials from which volatile organic compounds such as alcohols have been removed and which do not generate volatile organic compounds on practical use are desired in the related field since they are environment friendly.